On the right there are two blue disks, and two black circles. One circle is a little larger than the blue disks, the other much larger. The black circles exchange place every few seconds, and you may notice that the blue disks seem to change in size. This effect is moderately strong but fully illusory.
What to do
With the slider on the very right you can adjust the relative sizes of the blue circles. As soon as you move the slider, the automatic circle interchange stops and you can adjust at your leisure. If they look sufficiently equal, tap “ex” (for exchange) and the circles will switch places. In all likelihood, your careful adjustment will now look totally out of place. The number to the right of the “ex” button gives (in percent) the current size difference of the blue disks.
As Coren et al. (1976) put it “(This) is some sort of cognitive contrast effect, where in general the apparent difference in the size of the central test element and adjacent or surrounding elements is accentuated.” Helmholtz referred to it as “size contrast.” These illusory effects depend upon cognitive information processing strategies. Into the same class belongs the Ebbinghaus-Titchener illusion (as Coren et al. show with a factor analytical approach), but not size illusions which relate to depth perception / perspective. For me this illusion is rather fleeting: Its strength varies, sometimes it does not seem to be present at all. As such these cognitive size illusions are less “cognitively impenetrable” than the perspective-related size illusions.
If I find this illusion rather “mild” and open to cognitive modulation (and did not even read the original French publication), why did I go to the effort to include it in my collection? Because there is this titillating paper by Ittersum & Wansing (2012), which to me rather convincingly demonstrates that plate size influences the amount we eat – even nutritional experts fall for it. Plate size has increased over the last 100 years by ≈15%, and this may in part explain why most of us (well, me at least) eat too much.
So, in a cafeteria, choose a smaller plate!
Delboeuf, Franz Joseph (1865) Note sur certaines illusions d’optique: Essai d’une théorie psychophysique de la manière dont l’oeil apprécie les distances et les angles [Note on certain optical illusions: Essay on a psychophysical theory con cerning the way in which the eye evaluates distances and angles]. Bulletins de l’Académie Royale des Sciences, Lettres et Beaux-arts de Belgique, 19, 2nd ser. 195–216
Coren S, Girgus JS, Ehrlichman H, Hakstian AR (1976). An empirical taxonomy of visual illusions. Perception and Psychophysics, 20, 129-137 [PDF]